Why we're fat, Part 1: Simple question with many complex answers

Many think the answer is that we eat too much and don't exercise enough, but the reasons are more numerous and complex, say obesity researchers. And so are the solutions.

In the early 1970s, 14 percent of the adult population was considered obese, compared with 34 percent today. What's happened since then?

During the next four days, local and national obesity experts will weigh in with their theories. We'll serve up 40 reasons we're fat — and what you can do about them.

Today's list explores what we eat. Coming up: the influence of heredity, lifestyle and the environment.

The Food Pyramid: Established 30 years ago by the U.S. Department of Agriculture, the Food Pyramid led directly to the increased consumption of refined carbohydrates — a leading cause of obesity, say nutritional scientists. Wrongly accused, dietary fat was placed at the tiny tip of the pyramid, which pointed Americans toward eating more carbohydrates; a recommended seven daily servings of cereal, bread, pasta and the like sat at the base of the pyramid. Though the government has modified its stance and today promotes the My Plate guidelines, with far fewer carbohydrates, the die was cast, and the trouble began.

Fat phobia: As Americans reacted to the avoid-fat message, fat-free and reduced-fat products flooded store shelves. The consumption of sugar — a carbohydrate — skyrocketed. Sodas were fat-free, so Americans began to tank up. "The country's big low-fat message backfired," says Dr. Frank Hu, professor of nutrition and epidemiology at the Harvard School of Public Health. "The overemphasis on reducing fat caused the consumption of carbohydrates and sugar in our diets to soar. That shift may be linked to the biggest health problems in America today."

Carb consumption: Americans, on average, eat 250 to 300 grams of carbs a day, accounting for about 55 percent of their caloric intake. The most conservative recommendations today say they should eat half that amount. Some recommend far fewer. "If Americans could eliminate sugary beverages, potatoes, white bread, pasta, white rice and sugary snacks, we would wipe out almost all the problems we have with weight and diabetes and other metabolic diseases," says Dr. Walter Willett, chairman of the department of nutrition at the Harvard School of Public Health.

Dietary carbs convert to sugar in the blood and stimulate insulin, which moves sugar into cells, where it's stored as fat. "Over time, as our bodies get tired of processing high loads of carbs and insulin, which evolution didn't prepare us for, cells become more resistant to insulin," says Dr. Stephen Phinney, a nutritional biochemist and an emeritus professor at the University of California-Davis who has studied carbohydrates for 30 years. Insulin resistance leads to obesity, diabetes and other metabolic disorders.

Sugar consumption: "Sucrose and high-fructose corn syrup very likely contribute to insulin resistance," says Gary Taubes, science writer and author of "Why We Get Fat." "The more resistant your tissues are to insulin, the more you move calories into your fat tissues, causing you to burn less and store more." Most Americans consume three times more sugar than the USDA currently recommends.

The nature of fat: Like a tumor, fat tissue wants to grow, Taubes says. "It has an agenda of its own, and will grab available energy first." To expand, fat tissue snares the energy available from what you eat. This explains why the fat get fatter. Driving that agenda are hormones, mainly insulin and leptin.

Cheap food: Our food supply has become more plentiful, more varied and a lot cheaper, says Dr. Steve Smith, an obesity expert and scientific director of Sanford-Burnham Translational Research Institute for Metabolism and Diabetes, in Orlando. As a percentage of income, Americans spend 75 percent less on food today than they did in the 1960s.

Calorie consumption: Today, Americans eat on average 300 more calories a day than they did in 1985, and 600 more than in 1970, according to the USDA.

Bigger portions: Americans value two-for-one and super-sized products. Marketers know that, and have realized that amping up portions of cheap food is an inexpensive way to attract customers. Hence the 44-ounce Super Gulp. Twenty years ago, the average bagel was 3 inches wide and 140 calories; today it's 6 inches and 350 calories, according to the National Heart, Lung, and Blood Institute. Consumers still count the inflated bagel as one serving, though it's closer to three. Research shows that people eat more if more is presented to them.

Fewer breast-fed babies: Children who are breast-fed have lower rates of obesity, according to the American Academy of Pediatrics, which, along with many other health organizations, recommends exclusive breast-feeding for the first six months of life. However, according to the CDC's 2010 Breastfeeding Report Card, only 13 percent of American mothers breast-feed exclusively through six months.

Westernization: Obesity happens when populations become westernized, Taubes says. People who eat an ancestral diet don't have nearly as high an incidence ofdiabetes, cancer or heart disease as those living in Western societies do. "Go back to the 19th century, then add sugar and refined grains, and you get obesity," he says. "When you don't have those in your diet, you don't see those diseases."

What can we do about it? Start by modifying your diet to match the government's My Plate Guidelines, which recommend filling half your plate with fruits and vegetables, about a quarter with lean protein and a quarter with complex carbohydrates. Eliminate sugary sodas, sweets and refined starches, and be more mindful of portion sizes. Be aware of how food affects insulin levels and eat to keep insulin levels low and steady, so you control hunger and don't store fat. Eat more foods from the land, and fewer from a box.

Understanding why Americans are fat — and getting fatter — is as important as knowing what doesn't contribute. "It's not gluttony, and it's not lack of willpower," says Dr. Steve Smith, an obesity expert and scientific director of Sanford-Burnham Translational Research Institute for Metabolism and Diabetes, in Orlando.

"No scientist in the field will say the problem is strictly one of willpower," he says. "It's a result of the way our genes are interacting with an environment that is stacked against them."